Buggy code:
public class UndineConfig { private String lang ; private boolean enableAttachment ; private boolean enableSendFee ; private double sendFee ; private double attachFee ; private boolean enableSendSelf ; private List < String > disableWorldsToOpenAttachBox ; private int attachBoxSize ; private int maxAttachmentBoxCount ; private int uiEmptyLines ; private boolean enablePlayerList ; private int maxCreateGroup ; private int maxGroupMember ; private GroupPermissionMode sendModeDefault ; private GroupPermissionMode modifyModeDefault ; private GroupPermissionMode dissolutionModeDefault ; private int maxDestination ; private int maxDestinationGroup ; private UndineMailer parent ; public UndineConfig ( UndineMailer parent ) { } protected void reloadConfig ( ) { if ( ! ( parent . getDataFolder ( ) . exists ( ) ) ) { parent . getDataFolder ( ) . mkdirs ( ) ; } File file = new File ( parent . getDataFolder ( ) , "config.yml" ) ; if ( ! ( file . exists ( ) ) ) { if ( parent . getReleaseLang ( ) . equals ( "ja" ) ) { Utility . copyFileFromJar ( parent . getJarFile ( ) , file , "config_ja.yml" , false ) ; } else { Utility . copyFileFromJar ( parent . getJarFile ( ) , file , "config.yml" , false ) ; } } parent . reloadConfig ( ) ; FileConfiguration conf = parent . getConfig ( ) ; lang = conf . getString ( "lang" , "ja" ) ; enableAttachment = conf . getBoolean ( "enableAttachment" , true ) ; enableSendFee = conf . getBoolean ( "enableSendFee" , false ) ; sendFee = conf . getInt ( "sendFee" , 10 ) ; attachFee = conf . getInt ( "attachFee" , 10 ) ; enableSendSelf = conf . getBoolean ( "enableSendSelf" , false ) ; disableWorldsToOpenAttachBox = conf . getStringList ( "disableWorldsToOpenAttachBox" ) ; attachBoxSize = conf . getInt ( "attachBoxSize" , 1 ) ; maxAttachmentBoxCount = conf . getInt ( "maxAttachmentBoxCount" , 3 ) ; uiEmptyLines = conf . getInt ( "uiEmptyLines" , 2 ) ; <START_BUG> enablePlayerList = conf . getBoolean ( "enablePlayerList" , true ) ; <END_BUG> maxCreateGroup = conf . getInt ( "maxCreateGroup" , 5 ) ; maxGroupMember = conf . getInt ( "maxGroupMember" , 15 ) ; sendModeDefault = GroupPermissionMode . getFromString ( conf . getString ( "sendModeDefault" ) , MEMBER ) ; modifyModeDefault = GroupPermissionMode . getFromString ( conf . getString ( "modifyModeDefault" ) , OWNER ) ; dissolutionModeDefault = GroupPermissionMode . getFromString ( conf . getString ( "dissolutionModeDefault" ) , OWNER ) ; maxDestination = conf . getInt ( "maxDestination" , 10 ) ; maxDestinationGroup = conf . getInt ( "maxDestinationGroup" , 3 ) ; if ( ( sendFee ) < 0 ) { sendFee = 0 ; } if ( ( attachBoxSize ) < 1 ) { attachBoxSize = 1 ; } else if ( ( attachBoxSize ) > 6 ) { attachBoxSize = 6 ; } if ( ( maxAttachmentBoxCount ) < 1 ) { maxAttachmentBoxCount = 1 ; } if ( ( uiEmptyLines ) < 0 ) { uiEmptyLines = 0 ; } else if ( ( uiEmptyLines ) > 9 ) { uiEmptyLines = 9 ; } if ( ( maxDestination ) < 1 ) { maxDestination = 1 ; } if ( ( maxDestinationGroup ) < 0 ) { maxDestinationGroup = 0 ; } } public String getLang ( ) { } public boolean isEnableAttachment ( ) { } public boolean isEnableSendFee ( ) { } public double getSendFee ( ) { } public double getAttachFee ( ) { } public boolean isEnableSendSelf ( ) { } public List < String > getDisableWorldsToOpenAttachBox ( ) { } public int getAttachBoxSize ( ) { } public int getMaxAttachmentBoxCount ( ) { } public int getUiEmptyLines ( ) { } public boolean isEnablePlayerList ( ) { } public int getMaxCreateGroup ( ) { } public int getMaxGroupMember ( ) { } public GroupPermissionMode getSendModeDefault ( ) { } public GroupPermissionMode getModifyModeDefault ( ) { } public GroupPermissionMode getDissolutionModeDefault ( ) { } public int getMaxDestination ( ) { } public int getMaxDestinationGroup ( ) { } }

Fix:
enablePlayerList = conf . getBoolean ( "enablePlayerList" , false ) ;